It is well known that silicone rubbers formed from the vulcanization of polydiorganosiloxane fluids or gums alone generally have low elongation and tensile strength properties. One means for improving the physical properties of such silicone rubbers involves the incorporation of a reinforcing silica filler such as fumed silica, colloidal silica, silica gel, or precipitated silica into the fluid or gum prior to curing. However, such silicas have a tendency to interact with the polydiorganosiloxane fluid or gum causing a phenomenon typically referred to as "crepe hardening." A great deal of effort has been made in the past to treat the surface of reinforcing silica fillers with silicon compounds to make the surface of the silica hydrophobic. This surface treatment reduces or diminishes the tendency of the compositions to crepe harden and improves the physical properties of the cured silicone rubber. The present invention is a cost effective method for preparing hydrophobic precipitated silicas, which are particularly useful as reinforcing fillers in curable silicone compositions. In addition to being cost effective, the present method produces hydrophobic precipitated silicas which retain more of their structure, such as void volume, than hydrophobic precipitated silicas prepared by other methods known in the art.
Lentz, U.S. Pat. No. 3,015,645, teaches the making of hydrophobic silica powders by reacting an organosilicon compound such as dimethyldichlorosilane or trimethylmethoxysilane with a silica hydrogel in the presence of an acidic catalyst to form a hydrophobic silica hydrogel. The hydrophobic silica hydrogel in the aqueous phase is contacted with a water-immiscible organic solvent to convert the hydrophobic silica hydrogel to a hydrophobic silica organogel which segregates into the organic phase.
Burns et al., U.S. Pat. No. 5,708,069, teach a method for the preparation of hydrophobic silica gels under neutral conditions. The method comprises two steps, where in the first step the pH of a silica hydrosol is adjusted with a base to within a range of about pH 3 to pH 7 to facilitate formation of a silica hydrogel. In the second step the silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the silica hydrogel. Bums et al. teach that during conduct of the second step it may be desirable to add a surfactant or water- miscible organic solvent to facilitate the reaction of the organosilicon compound with the hydrophilic silica hydrogel.
Burns et al., U.S. Pat. No. 5,750,610, teach a method for preparing organosilicate-modified silica gels. The method comprises two steps, where in the first step an organosilicate-modified silica hydrosol is heat treated in the presence of a strong mineral acid at a pH less than about pH 1 to form an organosilicate-modified silica hydrogel. In the second step the organosilicate-modified silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the organosilicate-modified silica hydrogel thereby forming a hydrophobic organosilicate-modified hydrogel. Burns et al. teach that during conduct of the second step it may be desirable to add a surfactant or water-miscible organic solvent to facilitate the reaction of the organosilicon compound with the organosilicate-modified silica hydrogel.
Reinhardt et al., U.S. Pat. No. 4,072,796, describe a method where finely divided hydrophobic silica and silicates are prepared by precipitating of alkali silicate solutions with mineral acids or metal salt solutions and treating with organohalosilanes. The organohalosilanes are a prepolycondensed organohalosilane or a mixture of prepolycondensed organohalosilanes.
Parmentier et al., U.S. Pat. No. 5,009,874 describe a method for making a hydrophobic precipitated silica useful as a reinforcing filler in silicone elastomers. In a first step the precipitated silica in aqueous suspension is hydrophobed with an organosilicon compound. In a second step an organic solvent is added to effect granulation of the silica. The method is claimed to form hydrophobic, essentially spherical precipitated silica granules having a median particle size of at least 0.08 mm, a density of at least 0.15, a water wettability of at least 20%, and maximum water uptake of 5%.
Hydrophobic precipitated silicas prepared by the present method are useful in many applications such as reinforcing and extending filler in natural rubbers, thermal insulation, and as filler in flotation devices, they are particularly useful as reinforcing fillers in silicone rubber compositions.